Methods of making insulation, and products formed thereby



United States Patent 3,177,l07 f METHDS 0F MAKING INSULATN, AND PRODUCTSFSB 'HEREBY Michael J. Bolton, George A. Joyner, Jr., and Richard Lux,.all of Rome, Ga., assignors to General Electric Company, a corporationof New York Filed Dee. 28, 1961, Ser. No. 162,838 Claims. (Qi. 15d-M9)This invention relates to electrical insulation, and more in particularto an improved process for making electrical insulation from asbestossheet material impregnated with aluminum phosphate binder.

In United States Letters Patent 2,702,068 and 2,804,- 908, both assignedto the `same assignee as this invention, there are describedaluminum-phosphate-bonded asbestos insulating materials which providehard, dense laminated products adapted for high temperature electricalinsulating applications. We have found that by compressingaluminum-phosphate-impregnated asbestos at a particular stage in theprocesses for making the materials described in the above-indicatedpatents, the dielectric strength of such materials is greatly increased,while other desirable characteristics, such as high temperatureresistance, good mechanical properties, and low power factor, are notimpaired. Although compressing unimpregnated asbestos sheet material isa well known way of improving its mechanical handling properties, suchas flexibility, kwe have discovered that reducing the thickness of theimpregnated material within a critical range results in an unexpectedincrease in dielectric strength.

Accordingly it is an object of this invention to provide methods formaking aluminum-phosphatebonded asbestos products having increaseddielectric strength.

Another' object is to provide economical, hard, dense asbestos productsimpregnated with aluminum phosphate and having increased dielectricstrength.

Another object is to provide improved methods for increasing thedielectric strength of aluminum-phosphateimpregnated asbestos materialsby compressing the material.

Another object is to provide methods ,for increasing the dielectricstrength of aluminum-phosphate-impregnated asbestos sheet materials bycalendering the impregnated sheet material. -r

A further object is to provide improved methods for increasing thedielectric strength of laminates made from aluminum-phosphateabondedasbestos by reducing the thickness of the asbestos before'superimposedlayers are formed into a laminate. i

Other objects and advantages of the invention willi become: apparentfro-m y the specification, drawing, and

claims, `and the scope of the invention will be pointed out in theclaims.

Briey stated, according toone aspect of our invention,- hard, denseelectrical insulating asbestosproducts havingA high dielectric strengthmay be made by lprocesses. in which fibrous asbestos material isimpregnated with aluminum phosphate. The impregnated material isr thendried to substantially remove tackiness. After being dried, theimpregnated material iscompressed untilrits thickness is reduced about25 to 45 or until its density remains substantially constant 'on furthercompression. This significantly increases itsfdielectric strength, andthe thus treated material is suitable for use by itself as electricalinsulating material or as arlaminatcd product made as described` in thepreviously-mentioned patents.

In the drawing:

[FIGURE l is a graph showing the effect of lcompressively reducing thethickness of aluminumfphosphateice impregnated asbestos sheet materialon the dielectric strength of the material.

FIGURE 2 is a graph showing the effect of compresively reducing thethickness of aluminum-phosphate-impregnated Vasbestos sheet material onthe density of the material.

The general methods of making aluminum-phosphatebonded asbestosinsulating materials that our invention provides improvements in aredisclosed in United States Patents 2,702,068 and 2,804,908. Generallyspeaking, the methods involve impregnating iibrous asbestos sheetmaterial with a solution of mono-aluminum phosphate. Then theimpregnatedmaterial is air dried, generally at an elevated ltemperature untilsubstantially all of the tackiness is removed and the impregnatedmaterial attains suilicient strength for further handling. Our inventionis based on the ldiscovery that by compressing the material at thisstage in its processing, the dielectric strength of the .impregnatedmaterial is increased substantially. After the material has beencompressed, it may thenV be formed into laminates by superimposing aplurality of layers of material and compressing them at elevatedtemperatures. Or, the compressed material may then be coated with ahigher aluminum phosphate having the formula Alx(H3 XPO4)3 wherein x hasthe value of from about 1.3 to 1.7. The thus coated material may then besecurely bonded into a laminate as for example by winding on a mandreland heating up to about 260 C. for varying time cycles.V The laminatedmaterial may be coated with a suitable varnish, and it may be fired atan elevated temperature, as described in Patent 2,804,908.

The aluminum phosphates employed in the practice of this invention arewater'soluble or water dispersible products which, by removal of all orpart of the water, are converted yinto insoluble solid products havingexcellent bonding and insulating characteristics. The expressionaluminum phosphates as used in the specification and claims is intendedto :meancommercially pure aluminum phosphate or mixtures of aluminumphosphate and minor amounts of other Water soluble inorganic phosphates.

The term mono-aluminum phosphate is intended to mean aluminum phosphateof the formula'AMHgPOQa as well as the aluminum phosphates in which thealumi- -num to phosphate ratio varies slightly from the 1 to 3 ratio,and may be as high as 1.2 to 3. These aluminum phosphates are watersoluble materials which can be employed' in the form of relatively lowviscosity solutions for the complete impregnation of the fibrousasbestos, and they are characterized by the Ifact that after removal ofthe water, in whole or in part, there is obtained an impregnatedasbestos material of goed flexibility and high strength which can bereadily employed in the manufacture of laminated or molded products.

The higher aluminum phosphates employed for bonding themono-aluminum-phosphateimpregnated asbestos into a laminatedstructureare those which form a viscous aqueous solution having a viscosity whichmay be 100 times the viscosity of the monoaluniinum phosphate solution.These more viscous products, after removal of all or part of the water,are converted into relatively hard,

somewhat brittle, resin-likesolids and have been found e to haveexcellent bonding action for the mono-aluminum phosphate impregnatedasbestos.

The brous asbestos materials employed in the practice of our inventionmay be of any of the usual commercially available asbestos sheetmaterials consisting predominantly of asbestos fibers. Such sheetmaterial may consist entirely of asbestos liber or it may Vcontaincellulose ber, such as cotton or kraft, or jminor amounts of otheringredients such as starch, frequently present in commer cial asbestospaper for strengthening purposes. Also into increase its resistance totearing. ,Y

aluminum-phosphate-impregnated asbestos material afterv it has beendried suiiiciently to remove tack, the dielectric strength of the endproduct is greatly increased. They asbestos should be impregnated beforebeing compressed because pre-compressed asbestos is too dense to absorbthe mono-aluminum phosphate solution. The impregnated asbestos should bedried before being compressed to prevent it from sticking to thecompressing device and The preferred method of compressing a web ofsheet material is by passing it through calender rolls until thedesired.y amount of thickness reduction is attained. Depending on theforce applied by the rollers, several passes of the web through therolers may be required; Of course, other methods may be employed toreduce the material thickness, such as by compressing between theplatens of a press, or by compression molding.

Experiments have shown that there is a critical range for thicknessreduction that produces -optimum results. Test specimens were made fromsheets of commercially available asbestos paper having about 3 to 6%kraft fiber 42% because optimum increase lin dielectric strength isobtained between these limits. Y

FIG. 2r shows a graphin which the percent reduction in thickness ofaluminum-phosphate-impregnated asbestos sheet material is plottedagainst the density of the compressed material. The plotted data wasobtained from samples of the monoalurninurn-phosphate-irrrpregnated,Y

, compressed asbestos paper vdescribed above, before the paper wascoated with thehigher aluminum phosphate. It should be notedthatndensity levels olf between about Z55-40% thickness reduction, Withno appreciable increase.

in density being obtained with further reduction of mia- V terialthickness. VIt isV believed that the reasondensity `does not increasewith furtherl reduction of thickness is' thatthe material 'forming thesheet llows laterally rather Vthan being compressed further. Attempts toreduce .thickness above about 45 usually resulted in tearing the sheetmaterial. It is believed that the failure of the materialV to increasein dielectric strengthab'ove about 45 y thickness reduction is caused bythe bersfbeing crushed rather than merely being compressed. However, noexplanationA is known for the sharp increase indielectric. strengthafter Y about thickness reduction, since in this range on Theimpregnated paper was air dried for about 2 to 3minutes at` 130 to 150C. until substantiallyY all tackiness was removed and the paper attainedsucient mechanical strength for further handling. The

above characteristics were attained-when the diriedpimpregnated paperhad a moisture content of from 11 to 16% by weight, with optimumlresults being attained with moisture contents of from 412 to 14%. Thedried, impregnated Apaper' was ythen passed through calender rolls untilits thickness had been reduced predetermined amounts.r The pressure-onthe rolls varied from about 500 to about 3,000 pounds per lineal inch ofpaperwidth for various samples. The compressed paper was then coatedwith an aqueous solution of a higher aluminum:

phosphate of vthe typey described previously, andthen Wound on a mandrelto form a laminated cylinder. The f laminated cylinders were air driedon the mandrels at room temperature, and then the cylinders were removedfrom the .mandrels and oven dried at temperatures from about C. to about260 C. for varying time cycles.

During thenal drying cycle, the cylinders were coated with a suitablevarnish ofthe type described in United States Patent 2,702,068. y

Specimens were cut from the above-described cylinders and'tested by ASTMShort Time Dielectric Breakdown Method No. D-l49 to determine dielectricstrength. TheV test results are presented in the vform of a graph inFIG. l1.

' products are relatively inexpensive.

It will be understood, of course, that'while the forms The` .plottedvalues were obtained by averaging ther-testl results: for numerousspecimens. Specimens tested by standard -methods revealed thatmechanicalV properties,

scribed critical range. n f Y FIG. l reveals a critical frangeinwhich'the thickness 45% thicknessreduction. Below 25% thicknessreduction, the dielectric strength does not increase significantly.kBetween about 42% and 45 thickness reduction, `the `dielectric strengthbegins tofall off, and above about 45% thicknessreduction, any increaseis not worthwhile. The

preferred thickness reduction range is from about 32 tov 'reductionshould Vbe maintained. Thisrange .starts at z' around 25 thicknessreduction and extends to about the. density graph of FIG. 2 densityvaries uniformly linearly with thickness reduction.V Thus, the increasein dielectric strength between about 25 and/15% thickness reduction isboth unexpected and unexplainable by present theories. Y Y

Results similar to those shown in FIGS. 1 and 2 were obtained forspecimens made. from commercially available asbestos paperabout- 8 milsthick and for specimens made from paperY about 18 mils thickprocessed inthe kmanner described with reference to FIG.1. Specimens were made frompapers of widths Varying fromV V12 to 36 inches. Y Y

The electricalfinsulating products4 formed by our processes are usefulfor various electrical applications. They arev particularly well suitedfor use asinsulation in dry-type transformers. Without furthertreatment, the dried materials can be employed in sealeddry-typetransformers'as either cylinders on Which-fthe Winding coils arewrapped or as flat plates, spacers, end rings, and variousformsofvoltage barriers.` When employed in non-sealed dry-type"transformers, valuminum-phosphate-bonded asbestosyarticles shouldpreferably ybe rgivenva varnishing treatment with'any suitableelectricalinsulating varnish for. thepurpose ofy increasing waterresistance. The products formed by our processes are machinable,sawable, lsandable, and drillable, and the raw Imaterials, employed intheirmanufacture are relatively cheapysoV the resultin of the inventionherein shown and described constitute preferred embodiments of theinvention, it is not intended herein to illustrate all of the equivalentforms orramifications thereof;Y iti-.will also be understood thatr the.words used Vare Words of description rather than'of limitation, and thatvarious changes may be made without departing from` the. spirit or scopeof. the iuventionhereindisclosed, and it is airnedin the appended claimsVto cover all such f changes as fall withinthe true spirit and scope ofthe invention. y

What weclaimas.. new and desireto secure by letters Patent ofthe UnitedStates is:

l. TheY method of making a hard, dense .electrical in-l sultinglaminated asbestos product having high dielectricV strength, comprising:

(a) impregnating asbestos paper having a thicknessin therange-of aboutV8 to 18 milswith'an aqueous'solution of mono-aluminum phosphate, Y (b)drying the impregnated paper to substantially re- Vmove tackiness,l Y

(c) compressingthe dried l'paperuntil'itsthickness' is Y reduced aboutSiO-42%,

(d) coating the compressed paper with an aqueous solution of aluminumphosphate of the formula A1x(H3-XPO4)3,

(d) wherein X has a value of from 1.3 to 1.7, and

(e) forming a laminated product from the coated paper.

2. The method of making a hard, dense electrical insulating laminatedasbestos product having high dielectric strength, comprising:

(a) impregnating asbestos sheet material having a thicknessin the rangeof about 8 to 18 mils with aluminum phosphate,

(b) drying the impregnated sheet material to substantially removetackiness,

(c) compressing the dried sheet material until its thickness is reducedabout 32-42%, and

(d) forming a laminated product from the compressed sheet material.

3. The method of making a hard, dense electrical insulating asbestosproduct having high dielectric strength, comprising:

(a) impregnating brous asbestos sheet material having a thickness in therange of about 8 to 18 mils with aluminum phosphate,

(b) drying the impregnated material until its final moisture content isfrom 11 to 16% by Weight, thus rendering it substantially tack-free yetmechanically strong, and

(c) compressing the dried material until its thickness is reduced about30-42%.

4. The method of making a hard, dense electrical insulating laminatedasbestos product having high dielectric strength, comprising:

(a) impregnating asbestos sheet material having a thickness in the rangeof about 8 to 18 mils with alu- 5 minum phosphate,

(b) drying the impregnated sheet material until its final moisturecontent is of the order of l1 to 16% by Weight thereby to substantiallyremove tackiness, (c) compressing the dried sheet material until itsthickness is reduced about 30-42% and (d) forming a laminated productfrom the compressed sheet material. 5. A hard, dense laminated electricinsulating material formed of a plurality of sheets of asbestos breimpregnated with aluminum phosphate and thereafter individuallycompressed and densiiied after partial drying, said sheets being in therange of about 8 to 18 mils in thickness before impregnation and beingcompressed individually until sheet thickness is reduced about 30-42%.20

References Cited by the Examiner UNITED STATES PATENTS 1,780,743 11/30Codwise 161-268 25 2,463,856 3/ 49 Dickerman 156-281 2,601,243 6/52Botts et al 156--185 2,702,068 2/55 Spooner 161--205 2,804,908 9/57Spooner 117-126 30 ALEXANDER WYMAN, Primary Examiner.

EARL M. BERGERT, CARL F. KRAFFT, Examiners.

1. THE METHOD OF MAKING A HARD, DENSE ELECTRICAL INSULTING LAMINATEDASBESTOS PRODUCT HAVING HIGH DIELECTRIC STRENGTH, COMPRISING: (A)IMPREGNATING ASBESTOS PAPER HAVINGA THICKNESS IN THE RANGE OF ABOUT 8 TO18 MILS WITH AN AQUEOUS SOLUTION OF MONO-ALUMINUM PHOSPHATE, (B) DRYINGTHE IMPREGNATED PAPER TO SUBSTANTIALLY REMOVE TACKINESS, (C) COMPRESSINGTHE DRIED PAPER UNTIL ITS THICKNESS IS REDUCED ABOUT 30-42%L, (D)COATING THE COMPRESSED PAPER WITH AN AQUEOUS SOLUTION OF ALUMINUMPHOSPHATE OF THE FORMULA ALX(H3-XPO4)3, (D'') WHEREIN X HAS A VALUE OFFROM 1.3 TO 1.7, AND (E) FORMING A LAMINATED PRODUCT FROM THE COATEDPAPER.